Low temperature urea injection method

ABSTRACT

The invention relates to a method for injecting urea in an exhaust line of an engine ( 1 ), the urea containing ammonia which is used for chemically reducing, during a selective catalytic reduction reaction, or SCR reaction, the nitrogen oxides produced by the engine, the injection being performed upstream of a catalyst ( 3 ) in which the reaction takes place, the process comprising the following steps: the temperature in the exhaust line of the engine is measured ( 7 ); an amount of urea to be injected is determined ( 5 ), from a known relation between the temperature and the amount of urea to be injected for all possible operating temperature values; a determined urea amount is injected ( 6 ) in the engine exhaust line.

The present invention relates to a strategy for injecting urea into the exhaust line of an engine, and more particularly, of an engine installed in a diesel-type motor vehicle.

In diesel-type vehicle engines, fuel combustion results in the creation of gases such as nitrogen monoxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O).

These gases, which are generally known by the name nitrogen oxides (NOx), pose a hazard, firstly to human health, and secondly to the environment, since they help to create smog in cities and contribute to global warming by increasing the greenhouse effect. Consequently, we must come up with solutions to destroy these gases internally in vehicles before they are released into the atmosphere. The treatment of these gases in vehicles is strictly regulated, moreover, by various standards.

In order to chemically destroy these nitrogen oxides before they are discharged into the atmosphere, it has been envisioned to use a reduction process of the type known as SCR, or selective catalytic reduction.

Various reductants can be used for this purpose.

A first possibility, which is to use hydrocarbons as a reductant, has a double disadvantage in that it is both costly, due to the current price of fuel, and polluting, since it produces an increase in carbon dioxide emissions exiting the vehicle engine.

To remedy these disadvantages, numerous solutions have been proposed using urea as a reductant. That is, urea contains ammonia, which reacts with the nitrogen oxides in an SCR catalyst to form completely harmless dinitrogen. Most of these solutions involve injecting urea in liquid form. It is injected into the exhaust line at a temperature greater than 180° C.

At this temperature, the breakdown of urea into ammonia is complete and practically instantaneous, making it possible to use a relatively high injection rate, e.g., around 20 g/h. This high decomposition rate is partly due to the thermodynamic stability of urea and the size of the injected urea droplets.

The reason for using urea rather than pure ammonia is that ammonia is a toxic, corrosive gas, and therefore it is costly and complicated to provide a tank for storing a gas of this kind safely in a standard vehicle. Urea, on the other hand, may be stored as an aqueous solution, making it easier to store and inject into the exhaust line.

The reduction of nitrogen oxides using liquid urea involves several successive chemical reactions.

When a urea solution is injected into an exhaust line, first the water evaporates, thereby causing solid urea to form as tiny particles.

This reaction is expressed by the following chemical equation:

NH2-CO—NH2_((aqueous))−>NH2-CO—NH2_((solid)).  (Eq 1)

Generally, once evaporation takes place, the solid urea undergoes thermolysis in the surrounding high-temperature gases, at 180° C. and up. This thermolysis produces gaseous ammonia and isocyanic acid through the following reaction:

NH2-CO—NH2_((solid))−>NH3_((gas))+HNCO_((gas))  (Eq 2)

The last step of the reduction process is hydrolysis of the isocyanic acid to form gaseous ammonia and carbon dioxide:

HNCO_((gas))+H20_((gas))−>NH3_((gas))+C02_((gas))  (Eq 3)

Below 180° C., the breakdown of the urea particles by thermolysis occurs at a slower rate than the partial polymerization of urea. As a result, the urea particles turn into biuret. The biuret breaks down quickly by sublimation above 180° C., or below 180° C. on any catalytic surface containing acid sites. The two forms of biuret decomposition may be comprehensively described by the following reaction:

H2NCONHCONH2_((solid))+2×H2O_((gas))−>NH3_((gas))+CO2_((gas))  (Eq 4)

Now, it has been observed that in light vehicles, the temperature in the exhaust line is commonly below 180° C., in particular because of deceleration or frequent stopping of the vehicle, or prolonged low-speed city driving.

When the temperature of the urea injected into the exhaust line becomes less than 180° C., decomposition is no longer complete, and then complete polymerization of the high-concentration urea begins to occur, and a white solid forms on the surface of the SCR catalyst or the wall of the exhaust line.

As previously explained, the thermolysis of the solid urea, represented by equation 2, actually occurs because of the high temperature of the exhaust gases. When the temperature is lower, a solid polymer, cyamelide, is formed:

NH2-CO—NH2_((solid))−>HNCOX_((solid polymer))  (Eq 5)

This reaction occurs only with part of the urea that builds up at high concentrations on the surface of the catalyst, while the other part undergoes thermolysis as previously described. The formation of the solid polymer occurs at the moment where urea decomposition by thermolysis in the gaseous phase or by catalytic decomposition on the surface of the catalyst becomes slower than the reaction in which urea accumulates on the surface of the catalyst.

In this case, the surface of the catalyst is coated with a polymer, and the ammonia no longer comes in contact with the nitrogen oxides, which makes it impossible to reduce the latter. The decomposition rate of biuret decreases as a function of the temperature. The rate of urea polymerization, on the other hand, depends on the local concentration of urea or biuret molecules. That is, the higher the concentration, the greater the risk of polymerization, especially at low temperatures.

In addition, to destroy the polymerized solid deposited on the various elements of the exhaust line, the temperature must be raised to a value of about 450° C. But bringing the catalyst to such a high temperature on a regular basis is likely to destroy the active zones of the catalyst, which are indispensable for carrying out the chemical reactions of reduction.

A solution for limiting the accumulation of urea on the surface of the SCR catalyst and reducing the risk of polymerization would therefore be to control the quantity of urea injected into the exhaust line as a function of temperature.

In this way, the invention aims to remedy these disadvantages by proposing a urea injection method that can be used at any temperature, and particularly at low temperatures.

More precisely, the invention relates to a method for injecting urea into an exhaust line of an engine, the urea containing ammonia to be used in a selective catalytic reduction reaction—or SCR reaction—to chemically reduce nitrogen oxides discharged by the engine. The urea is injected upstream of a catalyst in which the reaction takes place, and the method comprises the following steps:

-   -   measuring the temperature in the engine exhaust line,     -   determining a quantity of urea to inject from a known relation         between the temperature and a quantity of urea to inject, for         all possible operating temperature values.     -   injecting the quantity of urea thus determined into the engine         exhaust line.

Such a method makes it possible, based on the temperature of the exhaust gases, to determine the quantity of urea that can be injected safely, i.e., with no risk of polymerization, which can result in poor removal of nitrogen oxides.

Thus, when the temperature is less than 180° C., we can choose to inject urea anyway, but at a lower concentration so that all of the urea injected will break down into ammonia, and there will be no polymerization.

A method in accordance with the invention makes it possible to determine a quantity of urea that can be injected into the exhaust line with no risk of polymerization, regardless of the temperature in the exhaust line. This quantity of urea is determined by using a relation that links the exhaust gas temperature to a quantity of urea over a range of values covering all of the temperatures that can be detected in the exhaust line.

This relation can be expressed as a mathematical relation, such as a polynomial equation, a graph, or a correspondence table.

These data are recorded in the memory of a processor, for example, to which the method refers in order to perform the various steps.

Preferably, data of this kind are determined experimentally, since they vary from one exhaust line to another. That is, they depend on physical characteristics of the various elements that make up this exhaust line, such as the engine type, the type of technology used for injection, and the catalyst type and size.

Even though the data used in calculating the quantity of urea are determined experimentally, the calculation can end up being skewed by parameters that vary in the exhaust line. Consequently, it can be useful at times to have the calculated quantities corrected during the process.

To this end, in some embodiments the method comprises the step of measuring the quantity of nitrogen oxides entering the catalyst, and using this measurement in the step where the final quantity of urea is determined.

Similarly, in some embodiments the method comprises the step of measuring the quantity of ammonia exiting the catalyst, and using this measurement to determine the quantity of urea to inject. That is, if there is too high a quantity of urea observed to be exiting the catalyst, there must be immediate intervention to avoid risking a noxious discharge of gases from the vehicle.

As a variant, a method in accordance with the invention also comprises one or more of the following steps:

-   -   the step of determining a maximum quantity of urea that can be         injected, based on the temperature, and determining the quantity         to inject as a function of this maximum quantity and at least         one other parameter that is calculated or measured in the         exhaust line,     -   the step of calculating the quantity of ammonia present in the         SCR catalyst, and using this calculation to determine the         quantity of urea to inject, and     -   the step of determining the quantity of urea to inject using         predetermined data, such as a data plot, recorded in a memory of         a processor used to carry out the method.

The invention also relates to a system for injecting urea into an engine exhaust line, the urea containing ammonia to be used in a selective catalytic reduction reaction—or SCR reaction—to chemically reduce nitrogen oxides discharged by the engine, the system comprising:

-   -   a catalyst which is the site of the reduction reaction,     -   a device for measuring the temperature of the gases in the         exhaust line, and     -   a device for calculating the quantity of urea to inject into the         catalyst, using a known relation between the temperature and a         quantity of urea to inject, for all possible operating         temperature values,     -   a urea tank and a urea injector located upstream of the         catalyst, and intended for injecting the quantity of urea         determined.

Several embodiments of the method will now be described in order to highlight other advantages and characteristics thereof. This description is given on a non-limiting basis, using the following figures:

FIG. 1 shows the change in certain parameters present in a vehicle engine over a known operating cycle, i.e., a MVEG cycle.

FIG. 2 shows a vehicle exhaust line equipped with a liquid urea injector,

FIG. 3 is a graph of a polynomial relation between a quantity of urea and a temperature,

FIG. 4 shows an operational diagram of various urea injection strategies, and

FIG. 5 shows the nitrogen oxide conversion rate as a function of temperature.

FIG. 1 shows various parameters measured in a motor vehicle engine over a standardized MVEG operating cycle. This cycle is currently used for vehicle certification in Europe.

FIG. 1 shows three curves representing the change in the vehicle speed (curve 11), the change in the quantity of nitrogen oxides discharged by the engine (curve 12) and the change in temperature (curve 13), respectively, as a function of time.

Operating the vehicle according to the MVEG cycle involves a succession of vehicle accelerations and decelerations. Thus, although the temperature in the engine would tend to increase over time, this increase is curbed by the frequent decelerations.

It can be observed that during the first 900 seconds of the cycle, the temperature does not exceed 180° C. except intermittently. Now, it has been previously explained that such a temperature is necessary for the urea to break down completely into ammonia with no polymerization.

Consequently, it seems obvious from this graph that it is not feasible to use a standard method of reducing nitrogen oxides with urea on light vehicles. Therefore, a method in accordance with the invention allowing low-temperature urea injection must be used.

An advantageous implementation of this method is used in an exhaust line as shown in FIG. 2.

In this figure, there is a vehicle engine 1 that releases nitrogen monoxide NO and nitrogen dioxide NO₂. At this engine's output there is an oxidation catalyst 2, used to increase the NO₂/NO ratio in the exhaust gases, thereby enabling better reduction of the nitrogen oxides subsequently in the selective reduction catalyst 3.

Lastly, the treated exhaust gases pass through a particulate filter 4 before being discharged into the atmosphere.

In order to make it possible to carry out the method, the various elements in the exhaust line are managed by the vehicle's onboard computer 5. For example, from the gas temperature reading taken by the device 7, the computer 5 is able to determine the quantity of urea that must be injected into the exhaust line by the injector 6, using experimental data recorded in a memory. The urea is stored in a tank 8.

In addition, the exhaust line is equipped with two gas detectors 9 and 10 to measure the quantities of gas present upstream and downstream, respectively, of the SCR catalyst 3. In an example, the computer 5 uses the measurements provided by these two detectors to control the injection of urea into the system.

The predetermined data contained in a memory of the computer 5 can be in plotted or table form, or any other data set.

For example, the relation between the quantity of urea to inject and the temperature can be modeled by a polynomial relation equal to:

Y=7E ⁻⁰⁶ x ³+0.0082x ²−2.5934x+182.2

This relation is shown graphically in FIG. 3 for temperatures varying from 100 to 200° C. This way, with the temperature value for the gases in the exhaust line, a vehicle computer can use this relation to determine the quantity of urea to inject, e.g., in the form of a liquid additive such as AdBlue. This quantity is expressed here in milliliters per hour.

To gain a better understanding of an injection method according to the invention, it will be described for three operating phases of the vehicle, which are distinguished here according to the exhaust gas temperature.

Range A corresponds to temperatures less than 120° C. It has been observed that it is useless to inject urea into the exhaust line at these temperatures, for several reasons:

-   -   firstly, at these temperatures, urea evaporates relatively         poorly, making it difficult to set off the first necessary         chemical reaction, which normally leads to urea particle         formation,     -   in addition, SCR catalysts are generally such that they cannot         be active at less than 120° C., and thus cannot serve as the         site for urea to break down into ammonia. Besides, even if the         SCR catalyst already contains ammonia stored in its micro-pores,         nitrogen oxide reduction is impossible at temperatures this low.

Consequently, in this vehicle operating range, urea injection is not undertaken.

Range C corresponds to temperatures greater than 180° C. It was explained above that at these temperatures, there is a complete and near-instantaneous breakdown of urea into ammonia. Consequently, in this operating range, it is possible to inject as much urea into the exhaust line as is necessary to reduce the nitrogen oxides released by the engine.

The specificity of the method according to the invention is evident in operating range B, which corresponds to temperatures between 120° C. and 180° C. Actually, in this temperature range, the hydrolysis reaction (Eq 3), which corresponds to a breakdown into the gaseous phase, cannot take place. Consequently, the breakdown of urea into ammonia is not complete, and thus it is advisable to limit the concentration of ammonia being injected.

The relation by which this concentration can be calculated is shown in an area B of FIG. 3. It must be determined for each type of engine, since it depends on a number of engine components such as injectors, the exhaust line, and the catalysts, e.g., the pollution control catalysts.

For an advantageous implementation of the invention, a processor may be used, installed for example in an onboard computer of a motor vehicle. This processor can use various parameters, which are predetermined, calculated or measured, in order to determine the quantity of urea that must be injected into the exhaust line. The role of these various parameters is illustrated in FIG. 4.

In this figure, a processor 20 is shown. This processor is in communication with a temperature measuring device 21. In an advantageous embodiment, this temperature device 21 is a thermocouple, i.e., a device comprising two metals connected by two junctions, and which generates a difference in potential that depends on the difference in temperature between the two junctions. In order to link the difference in potential to a temperature difference, one must know the thermocouple response as a function of temperature. This response is kept in a memory of the processor, for example, so that it can be used during a step of the method.

Thus, using the measured temperature and data 22 recorded in a memory of the processor, it is possible first of all to determine a maximum quantity of urea that can be injected safely into the exhaust line, as a function of temperature. The data 22 are in the form of one or more data plots, for example.

Next we determine the quantity of urea that actually needs to be injected in order to destroy the nitrogen oxides produced by the engine, which is a function of the maximum quantity of urea, the quantity of ammonia already stored in the SCR catalyst, the quantity of ammonia actually consumed by the nitrogen oxide reduction reaction, the temperature, and the speed.

Ammonia quantities are calculated by a calculator 23, as a function of the quantity of nitrogen oxides produced by the engine, and are measured by a detector 24.

To this end, the calculator uses the ammonia consumption rate, expressed by the formula:

Rate=k·A·(−AE/RT)·[A] ^(x) [B] ^(y) [C] ^(z) where:

-   -   K is a constant     -   A is an exponential factor,     -   AE is the activation energy,     -   R is the ideal gas constant,     -   T is the temperature,     -   A, B and C are the concentrations of the species in the         reaction, whose respective orders are x, y, and z.

The final quantity of urea thus determined is injected into the exhaust line by the injector 25.

FIG. 5 illustrates the change in this ammonia consumption in an SCR catalyst by showing the percentage of nitrogen oxide conversion as a function of the temperature and the ratio of nitrogen dioxide to nitrogen monoxide.

The quantity of ammonia already present in the catalyst is calculated based on parameters such as the quantity of urea previously injected, the quantity of urea consumed by the reduction reaction, and the storage capacity of the catalyst.

This storage capacity is for example determined experimentally based on the age of the catalyst. It also depends on the type of catalyst, i.e., whether one is using a catalyst with a micro-porous structure, a high-capacity catalyst, or a non-porous catalyst.

The ammonia used for nitrogen oxide reduction can be in any phase—liquid, gas or solid. However, as described in the present application, an additive such as AdBlue is preferably used, i.e., an aqueous solution containing 32.5% urea. For a standard diesel vehicle with a high capacity tank, the AdBlue injection rate is generally 20 to 40 liters per hour. The urea injection frequency can vary from 1 to 100 Hz, and a frequency of 10 Hz is preferably used. 

1. Method of injecting urea into an exhaust line of an engine, the urea containing ammonia to be used in a selective catalytic reduction reaction, or SCR reaction, to chemically reduce nitrogen oxides discharged by the engine, the injection taking place upstream of a catalyst in which the reaction takes place, the method comprising the following steps: measuring the temperature in the engine exhaust line, determining, based on this temperature, a maximum quantity of urea that can be broken down into ammonia and thus be injected into the engine exhaust line with no risk of polymerization, and injecting the quantity of urea thus determined into the engine exhaust line.
 2. Method according to claim 1, comprising the step of measuring the quantity of nitrogen oxides entering the catalyst, and using this measurement to determine the quantity of urea to inject.
 3. Method according to claim 1, comprising the step of measuring the quantity of ammonia exiting the catalyst, and using this measurement to determine the quantity of urea to inject.
 4. Method according to claim 1, comprising the step of calculating the quantity of ammonia present in the SCR catalyst, and using this calculation to determine the quantity of urea to inject.
 5. Method according to claim 1, wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method.
 6. System for injecting urea into an engine exhaust line, the urea containing ammonia to be used in a selective catalytic reduction reaction, or SCR reaction, to chemically reduce nitrogen oxides discharged by the engine, the system comprising: a catalyst which is the site of the reduction reaction, a device for measuring the temperature of the gases in the exhaust line, and a device for calculating, based on a measured temperature, the maximum quantity of urea that can be broken down into ammonia and thus be injected into the engine exhaust line with no risk of polymerization, and a urea tank and a urea injector located upstream of the catalyst.
 7. Method according to claim 2, comprising the step of measuring the quantity of ammonia exiting the catalyst, and using this measurement to determine the quantity of urea to inject.
 8. Method according to claim 2, comprising the step of calculating the quantity of ammonia present in the SCR catalyst, and using this calculation to determine the quantity of urea to inject.
 9. Method according to claim 3, comprising the step of calculating the quantity of ammonia present in the SCR catalyst, and using this calculation to determine the quantity of urea to inject.
 10. Method according to claim 7, comprising the step of calculating the quantity of ammonia present in the SCR catalyst, and using this calculation to determine the quantity of urea to inject.
 11. Method according to claim 2, wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method.
 12. Method according to claim 3, wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method.
 13. Method according to claim 4, wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method.
 14. Method according to claim 7, comprising the step of calculating the quantity of ammonia present in the SCR catalyst, and using this calculation to determine the quantity of urea to inject.
 15. Method according to claim 8, wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method.
 16. Method according to claim 9, wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method.
 17. Method according to claim 10, wherein to determine the quantity of urea to inject, predetermined data are used, which are recorded in a memory of a processor used to carry out the method. 